Ferrite magnetic powder and method for production thereof

ABSTRACT

A ferrite magnetic powder for bonded magnets bonded using a rubber-base resin as binder, which powder is a ferrite magnetic powder containing an alkaline earth metal and having a chlorine content of 0.05 wt % or less and a powder pH of less than 6. The magnetic powder is produced by pulverizing a calcinated product of ferrite composition containing an alkali earth metal, annealing the pulverized ferrite composition to relieve crystal strain, dispersing the annealed powder in water and adding to it a mineral acid, adding a dispersant thereto, and subjecting the dispersion to solid-liquid separation followed by vacuum drying to obtain a ferrite magnetic powder having a chlorine content of 0.05 wt % or less and a powder pH of less than 6.

TECHNICAL FIELD

This invention relates to a ferrite magnetic powder for bonded magnetsthat are bonded using a rubber-base resin binder and a method ofproducing the powder.

BACKGROUND ART

The magnetic characteristics of rubber-base bonded magnets that utilizerubber-base resin as binder are strongly affected by not only thecharacteristics of the magnetic powder used but also the filling rate ofthe magnetic powder in the rubber-base binder. Although the filling rateof magnetic powder in the rubber-base binder is affected by variousfactors, including the particle diameter and particle size distributionof the magnetic powder, the shape and surface configuration of theparticles and the type of rubber-base binder, what is basicallyessential is for the magnetic powder to have high compatibility with therubber-base binder without altering its inherent properties. In thisspecification, such property of a magnetic powder not degrading theintrinsic properties of a rubber-base binder is referred to as “magneticpowder non-reactivity” and the compatibility thereof with therubber-base binder is referred to as “magnetic powder affinity.”

When the magnetic powder non-reactivity and affinity are poor, viscositybecomes high and fluidity low at the time of kneading with therubber-base binder or at the time of forming a compound. As a result,the magnetic particles are subjected to mechanical stress. The strainthat arises in the magnetic particles owing to the application ofmechanical stress degrades the coercive force of the magnetic powder.

The non-reactivity and affinity of a ferrite magnetic powder can beassessed by, for example, measuring the viscosity and/or shear stress ofthe compound. The non-reactivity and affinity (compatibility) withrespect to the resin can be considered to be better in proportion as theviscosity and/or shear stress is smaller.

JP 2001-160506 A('506) teaches that a ferrite magnetic powder having acarbon content of 0.010-0.040 wt % and a pH of between 6 and less than 9can be obtained by pulverizing a calcinated product composed of ferrite,annealing the result to obtain a ferrite magnetic powder having a pH ofnot less than 9, suspending the powder in water and blowing carbondioxide gas into the suspension. The product is said to be excellent innon-reactivity and affinity with respect to resin-base binder. Reference'506 further teaches that in neutralization by adding a mineral acidinstead of blowing in carbon dioxide gas, coherence occurring in thedried product makes necessary a considerably strong crushing processand, in such case, total prevention of occurrence of internal stress isdifficult.

Japanese Patent No. 3294507 teaches that a ferrite magnetic powderhaving a carbon content of 0.015-0.080 wt % and a pH of between 7 andless than 10 can be obtained by bringing annealed ferrite magneticpowder into contact with a CO₂ source under stirring. The product issaid to be excellent in non-reactivity and affinity with respect toresin-base binder.

PROBLEMS TO BE OVERCOME BY THE INVENTION

One feature of the rubber-base bonded magnet is that its toughnessenables it to be mounted on equipment with a high degree of freedom. Forexample, a sheet-like rubber-base bonded magnet can be mounted at acurved portion in conformity with its curvature. This elasticity(toughness) of a bonded magnet may be degraded owing to properties ofthe magnetic powder. No reports have been published regarding causes onthe side of the magnetic powder for such molded component degradabilityexperienced by rubber-base bonded magnets.

In the case of rubber-base bonded magnets, when the molded componentobtained by shaping into a prescribed configuration experiencestoughness deterioration or change in shape, degradation of magneticproperties generally arises. In the case where, for instance, advantageis taken of the properties of a rubber-base bonded magnet to mount asheet-like molded component as bent along a curved surface, theperformance of the bonded magnet itself may be degraded if crackingshould occur. No measures for extending service life after bonded magnetmolding have so far been implemented from the side of the magneticpowder.

The present invention therefore resides in elucidating the cause on theside of the magnetic powder that produces degradation of magneticproperties after bonded magnet molding and to provide a ferrite magneticpowder for rubber-base bonded magnets that has been removed of thecause.

DISCLOSURE OF THE INVENTION

It was found that this object can be achieved by a ferrite magneticpowder containing an alkaline earth metal, which ferrite magnetic powderhas a chlorine content of 0.05 wt % or less and a powder pH of less than6. Such a magnetic powder can be obtained by pulverizing a calcinatedproduct of ferrite composition containing an alkaline earth metal,annealing the pulverized ferrite composition to relieve crystal strain,dispersing the annealed powder in water and adding a mineral acidthereto, and adding a dispersant thereto, and subjecting the dispersionto solid-liquid separation followed by vacuum drying. It was found thatthe coherence preventing effect was higher during vacuum drying when adispersant was added than when a dispersant was not added.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is diagram wherein change in viscosity and shear stress withshear rate in the case of a compound obtained by mixing a ferritemagnetic powder according to the present invention and a rubber-basebinder is compared with corresponding changes in the case of a ReferenceExample and a Comparative Example.

PREFERRED EMBODIMENT OF THE INVENTION

Ferrite magnetic powders vary considerably in composition and particleshape. The production process thereof by the dry method generallyincludes the steps of mixing starting materials palletizing →calcinating→ crushing → washing and dewatering →drying → crushing → annealingproduct. The final “annealing” step is for relieving the crystal strainthat arises during pulverizing after calcinating (and again duringcrushing following drying). The crystal strain arising during crushingand/or pulverizing needs to be relieved because it degrades the magneticproperties of the product, particularly its coercive force. Afterpassing through this annealing step, the ferrite magnetic powder has apH of 10-12, i.e., exhibits strong alkalinity. This rise in pH value isparticularly pronounced in the case of a ferrite magnetic powdercontaining an alkaline earth metal.

When the ferrite magnetic powder exhibits such strong alkalinity, itdegenerates the rubber-base binder and has a marked adverse effect onthe viscosity and fluidity of the compound. In addition, residualimpurity components and the like originating in additives and thestarting materials are detected in the ferrite magnetic powder subjectedto the annealing step. These residual components may become causes thatwork to degrade the rubber-base bonded magnet after kneading/molding.Among these components, chlorine was found to have an especially badeffect.

In this invention, the annealed ferrite magnetic powder is firstsuspended in water, preferably stirred thoroughly, and added with amineral acid. This treatment lowers the chlorine entrained by themagnetic powder to a substantially harmless level. Moreover, it wasfound that quality degradation of the molded rubber-base bonded magnetcomponent was prevented owing to lowering of the powder pH to below 6.The mineral acid used is preferably sulfuric acid.

Although treatment of lowering of the pH of the annealed powdersuspension using a mineral acid tends to promote coherence during thedrying after solid-liquid separation, it was found that coherence duringdrying can be avoided by carrying out an appropriate coherencepreventing treatment. Generally used treatments for preventing coherencethat might be considered include addition of an inorganic substancehaving little adsorbed water, addition of a fatty acid amide,fluorinated fatty acid or other such anti-sticking agent, and surfacetreatment with a silica type surface treatment agent or surfactant.However, the coherence preventing treatment according to the presentinvention is to prevent occurrence of coherence by adding a dispersant(surfactant) before the drying step and conducting the drying step underreduced pressure after solid-liquid separation.

By this, the chlorine content of the dried product obtained aftersubjecting the annealed ferrite magnetic powder to wet treatment inwater becomes 0.05 wt % or less, preferably 0.02 wt % or less, and thepowder pH becomes less than 6. The result is excellent in non-reactivityand affinity with respect to rubber-base binder and enables enhancementof the molded component bending strength. As a result, thecharacteristics of the rubber-base bonded magnet can be maintained overthe long term.

The pH value of the ferrite magnetic powder referred to here is thatmeasured in conformity with the measurement method of JIS K 5101.Although the ferrite magnetic powder to which the present invention canbe applied is not particularly limited as regards composition,application to a ferrite magnetic powder containing an alkaline earthmetal is particularly beneficial. The binder for bonding the ferritemagnetic powder is also not particularly limited insofar as it is arubber-base binder. It is, for example, possible to utilize avulcanizable rubber such as NBR (acrylonitrile butadiene copolymerrubber) or EPDM (ethylene propylene diene monomer rubber), or athermoplastic resin having rubber elasticity such as CPE (chlorinatedpolyethylene), plasticized PVC (plasticized poly vinyl chloride), or EVA(ethylene vinyl acetate copolymer). It is also possible to usechlorosulfonated polyethylene, silicone rubber and the like.

EXAMPLE Example 1

Iron oxide and strontium carbonate were weighed out and mixed at a moleratio of 5.75, the result was pelletized using water and, after drying,calcinated in a furnace at 1290° C. for 4 hours. The calcinated productwas coarsely crushed and then wet-pulverized using a wet mill to obtaina strontium ferrite magnetic powder of an average particle diameter of1.4 μm. The magnetic powder was annealed in a furnace at 980° C. for 1hour. The obtained annealed product contained 0.055 wt % of chlorine andhad a pH measured by the method of JIS K5101 of 10.22. The powder,12,000 g, was mixed with water to obtain a suspension with a pulpconcentration of 25 wt %. Sulfuric acid was added to the suspensionunder stirring so as to make the sulfuric acid concentration of thesuspension 0.10%, whereafter stirring was continued for 15 minutes.Water washing was conducted by decantation, Surfynol CT151 (product ofNissin Co., Ltd.) was added as dispersant in an amount to make theconcentration 0.25% relative to the ferrite magnetic powder. Stirringwas conducted for another 10 minutes. The result was dewatered and thecake obtained was dried under reduced pressure and then crushed using ahigh-speed agitation-type crusher to obtain a strontium magnetic powderof an average particle diameter of 1.4 μm. The obtained magnetic powderwas chemically analyzed. No more than 0.01 wt % of chlorine wasdetected. The pH was measured and found to be 4.9.

The obtained magnetic powder, 136.1 g, and 13.5 g of NBR were chargedinto a LaboPlast Mill (Toyoseiki Co.), kneaded for 10 minutes at atemperature of 80° C., once discharged, and kneaded again under the sameconditions. The obtained kneaded product was rolled into a sheet usingsix-inch rolls and the sheet was cut to obtain several samples measuring3 mm in thickness, 2 mm in width and 50 mm in length (for affinityevaluation) and a sample measuring 3 mm in thickness, 200 mm in widthand 50 mm in length (for molded component degradation evaluation).

Affinity was assessed by measuring shear rate (SR), viscosity and shearstress using a capillograph (Toyoseiki Co.). The results of themeasurement are shown in FIG. 1. The horizontal axis in FIG. 1 isgraduated for shear rate (sect⁻¹) and the vertical axis for viscosity(PaS) and shear stress (Pa), and shows change in viscosity and shearstress with change in shear rate. For comparison, FIG. 1 also shows asimilar evaluation of affinity for a magnetic powder at the annealingstage (which, as mentioned earlier, had a chlorine content of 0.055 wt %and a pH of 10.22; this magnetic powder being referred to as a ReferenceExample).

From the results shown in FIG. 1, it can be seen that, in comparisonwith the Reference Example, the magnetic powder of Example 1 exhibitedlow viscosity and shear stress values in the kneaded rubber-compoundproduct, as well as good affinity with the rubber-base binder.

Molded component degradability was assessed by maintaining the sheets at100° C. for five days, cooling them to room temperature, and thensubjecting them to a bending strength test in which each sheet waswrapped around a 22 mm diameter cylinder. Degree of cracking of thesheet surfaces was classified into three ranks. The sheet using themagnetic powder of this Example ranked A in degradability, while thatusing the annealing stage magnetic powder (Reference Example) ranked C.

A: No cracking observed whatsoever;

B: Cracks of under 3 mm length observed;

C: Cracks of 3 mm and greater length and 0.1 mm and greater widthobserved.

Comparative Example

Example 1 was repeated except that at the time of adding the sulfuricacid the sulfuric acid concentration was changed from 0.10% to 0.05%.The obtained magnetic powder was chemically analyzed. No more than 0.01wt % of chlorine was detected and the powder pH was 6.6.

The powder of this Comparative Example was subjected to the affinityevaluation of Example 1. The results are shown in FIG. 1. The powder wasalso subjected to the molded component degradability test of Example 1.It ranked B.

As explained in the foregoing, the present invention provides a ferritemagnetic powder that is superior in non-reactivity and affinity withrespect to a rubber-base binder. A rubber-base bonded magnet utilizingthe magnetic powder is excellent in molded component toughness and doesexperience cracking or the like even in a state subjected to bendingdeformation. As such, the present invention provides an outstandingeffect toward preventing loss of the deformability that is a feature ofrubber-base bonded magnets.

1. A bonded magnet obtained by bonding a ferrite magnetic powdercontaining an alkaline alkali earth metal and having a chlorine contentof 0.05 wt % or less and a powder pH of less than 6 with a rubber-basedbinder selected from NBR (acrylonitrile butadiene copolymer rubber) orEPDM (ethylene propylene diene monomer rubber).
 2. A bonded magnetobtained by bonding a ferrite magnetic powder containing an alkalineearth metal and having a chlorine content of 0.05 wt % or less and apowder pH of less than 6 with a rubber-based binder of a thermoplasticresin having rubber elasticity selected from CPE (chlorinatedpolyethylene), plasticized PVC (plasticized polyvinyl chloride) or EVA(ethylene vinyl acetate copolymer.